Biology 205 - Principles of Ecology Kilburn, p. 2kkilburn/205_lectures/eco6...Biology 205 -...

Biology 205 - Principles of Ecology Kilburn, p. 1Ecology of communities and ecosystems

I. General introductionII. Species abundance and diversity (Ch. 16)

A. Most species are moderately abundant in their communitiesB. Species diversity is a function of both number of species and their

relative abundanceC. Species diversity often increases with environmental complexityD. Species diversity is often greatest in communities with moderate levels of

disturbanceE. Environmental complexity and disturbance aren’t necessarily mutually exclusive

III. The trophic structure of communities: food webs (Ch. 17)A. Food webs summarize feeding relationships within a community; they provide a

basic and important description of community structure and function.B. The feeding activities of a few keystone species may control the structure of

communities, but their effects depend on a number of important variablesC. Exotic predators can collapse and simplify the structure of food webs.D. Humans can be keystone species

IV. Primary productivity and energy flow within an ecosystem (Ch. 18)A. Concepts and definitionsB. Terrestrial primary productivity is generally limited by temperature and moistureC. In aquatic systems, primary productivity is generally limited by nutrientsD. Consumers can increase or decrease primary productivityE. Energy flow within ecosystems results in trophic pyramids

V. Nutrient cycling and retentionA. Nutrient cycling is a key ecosystem service that is affected by human activity.B. The rate of nutrient cycling is affected by both biotic and abiotic factors.C. Nutrient cycling in streams is complicated by water flow

VI. Community succession and stabilityA. Succession is the gradual change in plant and animal communities in an area

following a disturbanceB. Community-level changes during succession include changes in species

richness and compositionC. Ecosystem-level changes during succession include changes in productivity,

respiration, biomass, and nutrient retentionD. Species turnover within a sere is a function of

1. biotic interactions among species2. extrinsic factors that influence composition of the pioneer community

E. Although details of species turnover vary among communities, both early and late successional species have common characteristics across communities

F. Stability in communities and ecosystems may be due to a lack of disturbance orto resisitance and resilience in the face of disturbance


I. General introduction

A. Some definitions:

1. Ecological community = an association of interacting species inhabiting a

defined area

a. communities actually studied are often subset of the “complete” community of an

area

b. subsets may be defined many different ways, depending on questions being

asked -- e.g.:

i. place/habitat: benthic vs. pelagic stream communities; soil communities

ii. taxon: small mammal community; invertebrate community

iii. guild or lifeform: shrub community; granivore community

iv. of course, combinations possible as well (benthic marine invertebrate

community; desert ant community; etc.)

2. Ecosystem = an ecological community and its abiotic components

a. note that distinction between ecosystem and community often fuzzy

i. most community ecologists incorporate components of the abiotic

environment in their concept of “community”

ii. few ecosystems ecologists would consider a termite mound with its

associated abiotic features an ecosystem (but it would fit the above)

b. common approach = consider ecosystem a higher level of organization:

i. = multiple communities linked by patterns of energy and nutrient flow

ii. e.g., forest, meadow, riparian communities within the same watershed

B. In this unit, we’ll move back and forth between these levels; much of what we’ll discuss

is applicable to both

1. First we’ll look at some basic features of community structure: species abundance,

diversity, trophic relationships

2. Then we’ll move on to important processes: productivity, nutrient cycling,

succession


II. Species abundance and diversity

One of the most obvious kinds of difference among communities is the abundance and

diversity of species in those communities -- what is species diversity, and why are some

communities more diverse than others?

A. Across communities, most species are moderately abundant; few are very abundant or

extremely rare

1. Pattern first documented by Preston (1948 +)

a. he looked at the relative abundance of species = relative number of individuals

by defining “abundance classes”, each of which was twice as “big” as the

one before it (i.e. -- abundance class 1 = 0-2 individuals; class 2 = 2-4

individuals; etc.)

b. then plotted frequency distributions: number of species in each community that

fall into each abundance class

c. resulting plots were almost always bell-shaped:this type of frequency

distribution called lognormal because it’s a normal distribution with one

logarithmic axis

d. pattern has several important implications:

i. indicate that most species are moderately abundant

ii. each community will have a very few highly abundant and extremely rare

species

iii. practical application: rare species will be hard to document unless

communities carefully sampled -- so serves as “reminder” to watch for the

rare ones

2. Pattern is interesting for several reasons

a. One of the best established, most consistent patterns in all of community

ecology, yet the underlying causes aren’t fully understood!

i. one possibility: statistical models indicate that this is the pattern we’d expect

if lots of random environmental variables were acting simultaneously on


many populations at same time

a) for a very few species, all variables would be positive => those would be

the few highly abundant species

b) for a very few species, all variables would be negative => those would be

the very few extremely rare species

c) for the rest, would have some positive, some negative effects leading to

moderate abundance

ii. May be the result of specific kinds of interactions (especially, some suggest,

interspecific competition)

iii. Or some combination of both -- still lots of debate about this

b. Regardless of underlying mechanism, pattern is very useful because it lets us

predict/estimate the number of species in a community even when we don’t

have a complete sample:

i. we can “plot” the abundances of the species in the sample

ii. this will give us part of the lognormal distribution; we can mathematically

extrapolate to “fill in” the rest of the curve

iii. because complete sampling often very difficult, this is extremely useful!

B. Species diversity is a function of both the number of different species in community and

their relative abundances

1. This idea is based on a largely intuitive sense that both factors are important --e.g.

(OH): two “communities” with different patterns of number, abundance:

a. both “communities” have two species

b. imagine walking through both communities -- in #1, you’re more likely to

encounter both species than in #2; we perceive that as more diverse


c. from a more ecological perspective, both species in Community 1 are likely to

have important effects on community; in Community 2, the rare species is likely

to have relatively little effect

2. So, ecologists recognize two components to diversity:

a. species richness = number of species

b. species evenness = relative abundance of species

3. These two components can be combined into a number of different mathematical

models of diversity, allowing quantitative comparisons among communities. One of

the most common is the Shannon-Wiener diversity index

a. where s = number of species

p = proportion of total # individuals represented by

each species

H’ = index of diversity

b. H’ will increase with both species number and with species evenness

c. e.g.: two communities, five species each with numbers of individuals as follows

(table 16.1):

Community A Community B

Species # Species #

1 21 1 5

2 1 2 5

3 1 3 5

4 1 4 5

5 1 5 5

d. work sample problem (end of notes)

e. note weakness: only one number used, so can’t tell how much of the difference

in diversity between communities is due to differences in richness and how

much to difference in evenness


Rank abundance curve

0.01

0.1

11 2 3 4 5

Abundance rank

Pro

po

rtio

nal

ab

un

dan

ce

Com. A

Com. B

4. Diversity can also be compared among communities graphically using rank-

abundance curves

a. plot log proportional abundance against abudance rank (from most abundant to

least)

b. e.g. from above table:

i. number of ranks = number

of species = richness

ii. slope reflects evenness:

more shallow = more even

iii. community 2 more diverse because more even

c. examples from actual studies: (OH, HO)

i. Caddisfly communities in Portugal: stream communities are more diverse

than coastal pond communities due to greater richness and greater

evenness

ii. Reef fish communities in the central Gulf of California are more diverse than

are communities from the northern Gulf primarily because of greater

evenness.

5. So, question now is why some communities are more diverse than others – this

question doesn’t have a simple answer, and ecologists are still working to figure it

out!

C. Species diversity often increases with increases in environmental/habitat complexity

1. Basic reasoning fairly intuitive:

a. to coexist, species need to occupy different niches

b. more complex environments offer more niches

2. e.g., famous studies by Robert MacArthur (and others) show that bird species

diversity increases with increasing forest complexity

a. first studied warblers coexisting in same forest; birds feed by gleaning insects

from bark, leaves

b. reasoned that the critical niche dimension was foraging zone in tree


i. predicted that coexisting spp. used different parts of the tree to forage

ii. prediction confirmed by observation

c. then extended reasoning to more general hypothesis about bird species in

whole forests:

i. quantified relationship between bird species diversity and vegetation stature

(volume of tree canopy) on Mt. Desert Island, Maine: increasing stature

increased the number of warbler species present

ii. compared relationship between species diversity and foliage height diversity

among 13 forest communities from North and Central America: found

positive correlation across communities

3. Similar results found in many studies of many kinds of animals in a variety of

communities -- but there are also lots of exceptions

a. note that in this kind of study, investigators need to identify appropriate elements

of environmental variability for the species they’re looking at: variation that

“matters” to one group of organisms might be irrelevant to another

4. Algae and terrestrial plant diversity also seems to be correlated with environmental

complexity/heterogeneity

a. Hutchinson (of n-dimensional hypervolume fame) articulated the “paradox of the

phytoplankton”): aquatic communities have high levels of plankton diversity in

spite of

i. plankton competing for same nutrients

ii. apparently uniform habitats

b. so heterogeneity doesn’t seem to explain phytoplankton diversity – and same

holds for terrestrial plant communities

c. In fact, though, soil, water chemistry (nutrients, etc.) do vary enough on a local

scale to help account for plant, algae diversity

i. Nitrate, silicate abundance in the surface waters of Pyramid lake, Nevada

show high heterogeneity (contrary to intuitive sense of surface waters as

being relatively uniform!)


ii. Nitrate and soil moisture highly variable over relatively small distances even

in an agricultural field (where plowing etc. tends to make soil more uniform)

d. Does heterogeneity also explain(at leat in part) high levels of terrestrial plant

diversity in tropics? Yes – because of subtle differences in soil characteristics --

e.g. from Jordan’s study

i. studied plant communities in Amazon forest

ii. identified six distinct communities (each with its own combination of

species) over a distance of <500 m and < 8m elevation, based on subtle

differences in soil properties (water, particle size primarily)

e. Both plant and algal species diversity decline with increased fertility (nutrient

availability)

i. e.g., Ghana rainforest: highest number of species found in soils with lowest

fertility

ii. Park Grass experiment in England: grass has been fertilized on

experimental plots since 1856; control plots left alone -- both species

richness and evenness have declined over time!

iii. same pattern has been found in many studies of diatoms

iv. seems surprising -- but actually makes sense:

a) increasing nutrient availability actually makes environments more uniform

(think of soil map from earlier)

b) with nutrient saturation, only limiting resource = light; one or a few

species will be able to overgrow others and exclude them (competitive

exclusion)

D. Species diversity is often greatest in communities with moderate levels of disturbance

1. Until now, have discussed habitats as though conditions changed relatively little;

conditions more or less stable and in equilibrium. In fact, all habitats are

susceptible to various kinds of disturbance

a. broadly defined as any discrete event that disrupts population, community or

ecosystem structure and function


b. note that an event that constitutes a disturbance to one population/community

might have no effect on another -- e.g., salt fluctuation would be disturbance to

coral community, but not to estuary

c. many different kinds of biotic and abiotic events/processes can cause

disturbance

d. we’ll talk about effects of disturbance again when we talk about stability and

succession; for now, we’ll just note that disturbances can range in severity by

ranging in

i. frequency (how often)

ii. intensity (how strong)

2. Connell (1975 +) was one of first ecologists to actually develop hypotheses about

community structure based on the assumption that disturbance would be the

norm (prior to that, most assumed equilibrial conditions made interspecific

competition “driving force” behind much of community structure/function).

3. Proposed “intermediate disturbance hypothesis” of species diversity: species

diversity will be highest under moderate levels (frequency or severity) of

disturbance: reasoned that

a. at high levels of disturbance, relatively few species would be present

i. disturbed habitats often have harsh physical/chemical conditions; relatively

few organisms have necessary physiological tolerances

ii. if disturbance frequent, relatively few species would have adaptations

necessary to colonize and complete life cycles before next disturbance

b. at very low levels of disturbance, relatively few species would be present

i. equilibrial conditions allow lots of interspecific competiton

ii. communities will become limited to a few excellent competitors

c. at intermediate levels, diversity will be greatest:

i. sufficient time between disturbances to allow many species to colonize

ii. not enough time for competition to reduce diversity

iii. also possible (depending on type of disturbance) that moderate levels don’t


produce conditions as severe as high disturbance

4. Test 1: disturbance and diversity in the intertidal zone (Sousa 1979):

a. studied invertebrate communities on boulders

b. disturbance = wave action overturning boulders, burying existing organisms and

exposing new surface for colonization by new individuals

c. measured force required to turn over boulders of different sizes in six study plots

and the frequency with which waves turned boulders over the course of two

years

d. using that information, classified boulders into low, moderate, and high

frequency disturbance classes and measured species diversity on each one

e. results conform to prediction:

i. majority of “high frequency disturbance”

boulders had only 1 species; none had

more than 5

ii. modal frequency for low disturbance was

2 species; none had more than 6

iii. intermediate disturbance had modal

frequency of 4 species, substantial

number had 5, 6, or 7

5. Test 2: disturbance and diversity in temperate grasslands (Whicker and Detling

1988)

a. studied temperate grasslands, where historically important disturbances have

been grazing, fire, and burrowing by mammals

b. focused on effects of prairie dogs (Cynomys) at Wind Cave National Park

i. squirrel relatives living in colonies of 10-55 individuals/ha

ii. as late as 1919, occupied 40 million ha of NA grasslands; now reduced to

tiny fraction of that (populations down to about 2% of original)

iii. build extensive burrow systems: 1-3 m deep, 15m long, requiring removal

and dumping of 200 kg of soil, usually deposited as mound 1-2m diameter


around the entrance

c. found that prairie dog colonies had vegetation communities much different from

surrounding (undisturbed) prairie, with greatest diversity at intermediate levels of

disturbance (= intermediate densities of prairie dogs)

i. “bare surface” of newly-excavated soil good habitat for good colonizers

ii. over time, early colonizers displaced by better competitors

iii. at intermediate levels of burrowing, get complex mixture of “patches” of

different age, plant composition (old ones with good competitors, young

ones with good dispersers)

E. Prairie dog example illustrates that environmental complexity and disturbance aren’t

necessarily mutually exclusive; in many communities, disturbance creates the

environmental complexity/heterogeneity that promotes species diversity

1. In forests, e.g., blow-downs create open patches that are structurally and

functionally distinct from closed-canopy patches

2. In grasslands, fire and grazing important for keeping “best competitors” from

excluding other species

3. conservation implications:

a. if we’re trying to restore a community or ecosystem and want to preserve

optimal species diversity, may need create or simulate appropriate disturbance

conditions

b. also means that, for some communities, moderate human use (e.g., logging,

grazing) may still be compatible with conservation goals -- just need to be

careful.

4. ON YOUR OWN: Read about human disturbance and diversity of chalk grasslands

(pp. 388-39=89); what evidence suggests that high levels of diversity result form

human activity? Answer review question #8 p. 390.

III. The trophic structure of communities: food webs

A. Food webs summarize feeding relationships within a community; they provide a basic

and important description of community structure and function.


1. This is a huge and complex area of ecology – we’re just going to introduce a few

basic concepts and identify a few common patterns without going into detail.

2. The earliest work on food webs (1920's) was qualitative and focused on relatively

simple communities; we’ll use an example to illustrate basic vocabulary:

a. autotrophs vs. heterotrophs

b. producers vs. consumers

3. Even in relatively impoverished communities, trophic relationships complex; when

studied in more detail, level of complexity becomes huge

a. Winemiller’s study (fig. 17.4) of food web consisting only of 10 of the most

common species in a community of 20 fish species (leaving out all other

species!) -- clearly, huge amount of complexity!

b. level of complexity can be reduced several ways:

i. aggregation: combine species according to taxonomy, trophic position

e.g., food web of just species at soil surface of desert in the Coachella

Valley, CA: 174 species of plants, 138 species of inverts, 55 arachnids, over

2,000 species of microorganisms reduced to (relatively) simple diagram:

ii. focusing on interaction

strength (measured, usually,

by amount of energy flow), with

or without removing “weak”

links (those involving less than

some threshold value of energy

flow).

a) e.g., simplified Winemiller

web easier to follow

b) e.g. Phragmites food web:

by focusing on interaction

strength, can make better

predictions about the


effects of different species in the web -- e.g., blue tits have strongest

effects along “left side” of web

4. Food web structure can be quantified so that comparisons can be made among

communities

a. three factors commonly quantified; all relate to complexity:

i. connectance = measure of relative complexity = actual number of

“connections”/total possible connections (e.g., Coachella has connectance

of .49 -- fairly high)

ii. linkage density = # of links/species (# links/# species)

iii. chain length = # of links between producer and top consumer (often

averaged for entire web)

b. unfortunately, although tons of studies have been done comparing communities,

general patterns have been very elusive

i. major problem is with the data themselves:

a) virtually all published webs involve some sort of aggregation; kind and

degree can make big difference in resulting patterns (in one study, Paine

showed that connectance values ranged from ~ .30 to .60 for

same community studied by different investigators!)

b) even carefully detailed webs are often over-simplifications of actual

relationships:

i) food habits (position in web) may change over time (because of

changes in organism, changes in environment)

ii) migrating species may be very important elements of web at some

times, but absent at others

iii) strengths of interactions almost never known

ii. one pattern that seems to hold with at least some consistency is that chain

length seldom exceeds 4 -- but reasons unclear!

a) early hypothesis was that chain length was related to productivity (amount

of energy at “bottom” of web);


b) Pimm et al. disproved by experiments on tree-hole communities

supplemented with external energy sources: no change in chain length

over 4-fold increase in energy inputs

iii. one pattern that doesn’t hold is relationship between community stabilityand

food web complexity

a) this was early hypothesis of MacArthur and others

b) idea was that communities “connected” by complex food webs wouldbe

less susceptible to disturbance and/or recover from disturbance more

quickly than would simpler communities

c) field, laboratory and modeling studies fail to bear this out -- but why isn’t

really known

5. Conclusion: food web structure, even when qualitative, does give us important

insights into community function; although general patterns still not clear, very active

area of research

B. The feeding activities of a few keystone species may control the structure of

communities, but their effects depend on a number of important variables

1. We’ve mentioned keystone species already (Paine’s starfish Pisaster); now we’ll

define and distinguish from dominant species:

a. keystone species = species that have large impacts on their community

structure in spite of low biomass

b. dominant species = species that have large impacts on their community’s

structure because of their large biomass

2. Pisaster = classic example: because of its trophic relationships with other tide pool

invertebrates, it largely controlled species diversity within tidepools

3. Jane Lubchenko (1978) was one of first investigators to demonstrate that theeffects

of keystone herbivores on trophic structure depended on several important

factors, and that those effects can vary among communities.

a. also studied tidal communities, but worked on snail-algae interactions

i. snail = Littorina littorea, a grazer on a variety of tidepool macroalgae


ii. previous studies of the effects of herbivores on species diversity had mixed

results: some found herbivory increased diversity, some found it

decreased diversity

b. proposed that three major factors interacted to determine the effect of

keystone species on trophic structure:

i. food preferences of the consumer (herbivore)

ii. competitive interactions among the plant species

iii. variation in both food preference and competition across environments

c. did a series of experiments in lab and field:

i. in lab studies, demonstrated that Littorina had definite food preferences

(Enteromorpha > Chondrus)

ii. in field studies, found strong relationship between presence of snail and

densities of algae:

a) where snail present, preferred algae absent and least preferred algae

present

b) where snail absent, preferred algae outcompetes least preferred algae

(preferred species is best competitor)

c) so snail has effect on algae similar to Pisaster on prey species:

consumption reduces competitive exclusion of least preferred by most

preferred

d. Found that species richness in tide pools related to snail density: highest

species richness in tide pools with intermediate snail densities

i. at low snail densities, preferred alga outcompetes others

ii. at high snail densities, snail eats most species; least palatable only onesthat

persist

iii. so, in tidal communities, effect of herbivory is to increase species diversityat

intermediate herbivore densities (herbivory ~ disturbance)

e. Found different effect of snails on algal diversity in emergent (above-water)

habitats:


i. different combination of species present

ii. preferred prey species = least competitive of species present

iii. increasing snail density decreases algal species richness across all

densities: effect of snail is to speed up competitive replacement

f. conclusion from these and other studies:

i. consumers can definitely be keystone species in a wide range of

communities/ecosystems

ii. effects of consumers on food webs depends on several factors, including

a) consumer food preferences

b) local population densities of consumers

c) relative competitive ability of prey species

iii. because a-c above can vary over time and among environments, effects of

keystone consumer can also vary over time and among communities

C. Exotic predators can collapse and simplify the structure of food webs

1. In our earlier discussion about competition, we noted that the introduction of exotic

species can reduce biodiversity through competitive replacement

2. another major problem with exotics (probably even greater than #1) is that they

reduce biodiversity through their effects on food webs: net effect is reduction insize,

complexity of food webs

a. e.g., Nile perch in Lake Victoria

i. historically, Lake Victoria had over 400 species of fish, many endemic

ii. introduction of Nile perch and Nile tilapia around 1954; population really

exploded beginning in 1980's (possibly as consequence of other changes in

surrounding environments)

iii. fish catch now dominated by only 3 species: perch, tilapia, and one native

sp.

iv. effect of perch may well be largest mass extinction of vertebrates in modern

times!

b. unfortunately, similar processes happening all over, especially with introduction


of game fish in freshwater systems

c. ultimate consequences unclear: we don’t know whether or not changes in trophic

structure will have effects on communities beyond the obvious reduction in

biodiversity

D. Humans can act as keystone species – e.g. from tropical South America (Kent

Redford)

1. Amazonian subsistence hunteres take ~ 1.4 million mammals + 5 million birds and

reptiles annually

2. Commercial hunters take ~ 4 million (for meat, skins, feathers, etc.)

3. Both activities produce “indirect kills”; Redford estimates ~ 60 million killed annually

4. Is this an example of keystone predation? One test = examine which species are

affected: usually, large animals are disproportionately hunted

a. at one national park in Peru, the 18% of mammal species actually hunted

constitute over 75% of available mammal biomass

b. similarly, the 9% of bird species actually hunted constitute over 52% of bird

biomass

c. total hunting impact = 80-93% reduction in mammal biomass, 70-90% of bird

biomass

d. so clearly humans have an effect disproportionate to our own biomass!

5. ON YOUR OWN: are the animals that humans hunt keystone species? What’s the

evidence? (pp. 407-408).

6. ON YOUR OWN: read section on keystone predators and pest control (pp. 408-

410); be able to describe the basic elements and interactions of the citrus ant food

web and the role of ants as pest control agents.

V. Primary productivity and energy flow in ecosystems

A. Concepts and definitions

1. In this unit, we’ll look briefly at how energy “moves” through ecosystems, with special

attention to

a. factors affecting the amount of energy available within systems


b. effects of energetics on trophic structure

2. Energy for most ecosystems comes from the sun:

a. Primary productivity = solar energy converted by plants to chemical energy in

the form of fixed carbon

b. Gross Primary Productivity (GPP) = total amount of solar energy converted

(or total amount of carbon fixed, as surrogate measure)

c. Net Primary Productivity (NPP) = total amount of chemical energy (fixed

carbon) left after plant respiration

i. = GPP - energy “burned” by plant for own uses

ii. = amount of energy available to “fuel” the rest of the system

3. Primary productivity is influenced by multiple factors:

a. Abiotic factors affect physiological processes in individual plants (including

PSN) -- often referred to as “bottom-up control” of PP

b. Biotic factors affect plant population size, which in turn affects total PSN -- these

often referred to as “top-down control” of PP

4. To evaluate the effects of energy on trophic relationships, organisms are assigned

to trophic levels based on position in food webs relative to plants – e.g.

a. plants = producers

b. plant-eating animals = primary consumers

c. animals eating plant-eating animals = secondary consumers

d. etc. – up to ~ 4' consumers (almost never more – why?)

B. Terrestrial primary productivity is generally limited by moisture, temperature

1. Early investigation by Rosenzweig (1968) related PP to annual

evapotranspiration

a. AET = total amount of water that evaporates and transpires (from plants) off a

landscape in one year

b. AET will increase with both precipitation and temperature

c. (OH): found very strong, positive relationship between PP an AET: productivity

is highest in warmest, wettest environments (think about why)


2. Although relationship to AET is very strong, there’s still some variation in PP not

explained by variation in AET (i.e., within a given AET level, can still get differences

in PP)

a. subsequent experimental work shows additional correlation between PP and

soil fertility (nutrient availability)

b. not surprisingly (given how we treat our lawns, gardens, and agricultural fields!),

nitrogen and phosphorous are generally the important nutrients

C. In aquatic systems, primary productivity is generally limited by nutrients

1. For freshwater systems (especially lakes), there’s a very strong and well-

documented pattern of increasing PP with increasing nutrient levels –

a. increasing nutrient concentrations -->increases algal biomass -->increases

NPP

b. for freshwater systems, phosphate is most often the critical nutrient

2. In marine systems, PP is highest along continental margins

a. these areas have the greatest nutrient concentrations as a consequence of

i. runoff from terrestrial systems

ii. mobilization due to disturbance of bottom sediments

iii. upwelling

b. for marine systems, nitrogen seems most often to be the limiting nutrient

D. Consumers can increase or decrease primary productivity

1. Consumers (primary or higher) affect NPP via their effects on food webs – similar

to the effects of keystone species, but the effects are on productivity rather than on

species diversity

2. E.g. #1: piscivorous fish depress primary productivity in some lake systems

a. Important trophic links are: piscivorous fish -->planktivorous fish -->large

herbivorous inverts -->plankton

b. increasing piscivore abundance ultimately decreases plankton abundance;

decreasing piscivore abundance has opposite effect

3. E.g. #2: Moderate densities of grazing increase productivity in Serengetti


grasslands

a. grasses exhibit phenomenon of compensatory growth

i. = increased growth rate following grazing

ii. results from effects of grazing on leaf area, shading, and other properties

b. at moderate grazing levels, NPP is greatest

i. at low levels, no compensatory growth occurs

ii. at high levels, grazing damage is too severe for plants to compensate

iii. at intermediate levels, get net increase in PP because compensatory growth

gives net gain in grass biomass

E. Energy flow within ecosystems results in trophic pyramids

1. In any ecosystem, only a very small fraction of incoming solar radiation is

“converted” by plants

a. e.g., Hubbard Brook -- NPP = ~ 1% of total incoming solar radiation

b. means that all other food webs are based on only tiny fraction of total energy!

2. As energy is “transferred” through the food web, only a relatively small amount is

actually available for transfer from one trophic level to the next

a. review energy use in organisms: only a very small amount is used for the

production of biomass (growth, reproduction), but that’s the only energy

available for transfer to next trophic level

b. in addition to energy “lost” at each level, energy is also lost in the transfer from

one level to another due to inefficiency of assimilation (think about “waste” not

consumed)

c. the general rule of thumb is that no more than ~10% of the energy available in

one organism (or one trophic level) will be available to the next

d. the consequence is that when the total energy content of each trophic level is

calculated and plotted, get trophic pyramid:

i. generally, trophic pyramids have 4 levels or less (remember chain length!)

ii. consider implications for conservation of top predators!


V. Nutrient cycling and retention

A. Nutrient cycling is a key ecosystem service that is affected by human activity.

1. The major biogeochemical cycles (e.g., C, N, P) make key nutrients available to

organisms; they depend on complex interactions between abiotic and biotic

components of the environment.

2. The carbon cycle is our source of structural carbon:

a. The major pool of activlely-cycled carbon is atmospheric CO2.

b. Although most carbon is rapidly cycled, some becomes sequestered for long

periods of time in the forms of peat, fossil fuels, and carbonate rock.

c. Overview of carbon cycle (fig. 19.4)

d. The major anthropogenic effects on the carbon cycle are

i. burning fossil fuels, increasing atmospheric CO2

ii. large-scale deforestation has the same effect (directly by burning and/or

decomposition of wood; indirectly by reducing PSN)

e. The main consequence of concern is effects of increasing CO2 on global

climate -- see the IPCC climate change report for possible consequences.

f. Changing CO2 levels may also have complex consequences for everything from

individual plants (carbon availability affects energy allocation patterns, which can

lead to changes in life history) to whole ecosystems (as atmospheric carbon

levels change, competitive relationships among plants may change) – very hot

topic!

3. The nitrogen cycle provides nitrogen for proteins, nucleic acids

a. The major source of cycled N is N2 gas in the atmosphere.

b. The N cycle is complex:

i. consists of “inner” and “outer loops”: cycling among organisms and cycling

between organisms and the atmosphere

ii. uptake by primary producers requires complex chemical conversions fromN2

gas to ammonia, nitrite, nitrate; conversions require specialized organisms

(bacteria primarily)


c. note that nitrate is negatively charged; nitrate not assimilated by plants will be

rapidly leached from soils

d. Overview of the N-cycle

e. major anthropogenic effects include

i. burning fossil fuels returns N to atmosphere as nitrous oxides -- this is a

greenhouse gas and a source of acid precipitation

ii. deforestation precipitates nitrogen loss from forest systems:

a) after trees cut, N returned to soils through decomposition

b) nitrifying bacteria convert to nitrate

c) with less vegetation to assimilate nitrate, it’s leached from the soils and

lost via streams, rivers

iii. use of nitrogen fertilizer increases nitrogen availability in aquatic systems,

leading to nutrient pollution, eutrophication, changes in algal species

diversity, and fish kills

4. The phosphorous cycle provides P for structural compounds (bone), ATP, and

nucleic acids

a. in contrast to C, N cycles, the P cycle is mostly sedimentary: the major pool of

phosphate is in phosphate-bearing rock/sediment

b. cycle is relatively simple, with release via erosion and simple cycling among

organisms

c. cycle is complicated by the chemistry of phosphate: pH limits its availability in

both aquatic and terrestrial systems:

i. at pH < 5.5 and > 7.0, phosphate combines with iron, aluminum (low pH),

calcium (high pH) and other elements to form insoluble compounds

ii. only soluble compounds can be assimilated by plants

d. overview of cycle

e. major anthropogenic effects:

i. acid precipitation/mine drainage may have long-term effects on phophate

availability in affected systems


ii. mining phosphorous-bearing rock for fertilizer harms local environments

iii. use of phosphate-rich fertilizers has same effects as using nitrogen fertilizers

B. The rate of nutrient cycling is affected by both biotic and abiotic factors.

1. Rates of nutrient cycling depend on rates of decomposition

a. decomposition converts nutrients from organic to inorganic form

mineralization); assimilation usually requires inorganic forms of major mineral

nutrients

b. decomposition rates increase with AET

c. in both terrestrial and aquatic systems, decomposition rates depend on

chemical composition of litter; in general

i. rates increase with increasing N content

ii. rates decrease with increasing lignin content

2. Plants and animals have complex effects on nutrient cycling and availability –e.g.

a. prairie dogs (and other burrowing mammals) increase the nitrogen content of

the grasses on their colonies (through complex processes)

b. large grazers speed up turnover of plant biomass in grassland systems; this

requires an increase in decomposition

c. leguminous plants increase soil nitrogen:

i. directly through action of root symbionts and indirectly through the production

of nitrogen-rich detritus

ii. changes in soil nitrogen availability can have effects on plant community

structure

iii. cause of concern when exotic legumes imported for, e.g., erosion controlinto

“naive” communities (Africa, Hawaii, eastern US)

C. Nutrient cycling in streams is complicated by water flow

1. In contrast to terrestrial systems (where nutrients are retained on site in soils),

nutrients in streams may be transported from their “place of origin”

2. The pattern of uptake, release, downstream movement, reuptake, etc., is better

described as a spiral than as a cycle


3. Spiraling dynamics are summarized in a quantity called spiraling length (S)

a. S = VT where

i. S = spiraling length

ii. V = velocity of nutrient flow (not necessarily the same as the velocity of the

stream!) downstream

iii. T = time for nutrients to complete one cycle of uptake/release/reuptake

b. so, S is low when V, T are small and

c. low S implies high nutrient retentiveness (nutrients are staying in one place for

longer) within a stream

4. Factors that affect nutrient retention will affect S (and vice-versa) – e.g., study by

Grimm on aquatic invertebrates in Sycamore Creek, AZ (fig. 19.14):

a. System basics:

i. like many streams in arid southwest, this stream has high densities/biomass

of aquatic invertebrates

ii. > 80% of macroinvertebrate biomass = organisms in the category of

“collector-gatherers” = detritus feeders

iii. as in many systems, primary productivity in this stream is limited by N

availability – so N retention is of particular interest

b. Grimm found that macroinvertebrates increased N retention:

i. quantified N budgets (measured amount of N at each level and the rate of

movement among trophic levels)

ii. measured N retention as the daily difference between N inputs and outputs

iii. set daily difference at 100% of budget & expressed components as

proportion of that 100% – found that

a) ingestion rates = 131% (macroinvertebrates, like many other

detritus/plant feeders, consume feces to improve extraction of nutrients)

b) 15-70% of nitrogen excreted and recycled in the form of ammonia –

these high feeding and excretion rates reduce T (fast cycling)

c) ~ 10% of N at any one time is in the biomass of the invertebrates (which


stay in place) – this reduces V (rate at which nutrients are transported)

d) so together, inverts reduce V, reduce T, and therefore reduce S: short

spiraling length = increase nutrient retention

D. ON YOUR OWN: read Plants and Nutrient Dynamics of Ecosystems (pp. 443-444);

what effects have exotic legumes had on nutrient availability (and how); what are some

of the potential consequences of those changes (think about what you know about

species interactions)?

VI. Community succession and stability

In this unit, we’ll look at how communities change following disturbance. Those changes

have effects both at the community and at the larger ecosystem scale; we’ll introduce the

ecosystem changes here, then look at those processes in more depth in the next units (as

time permits)

A. Succession is the gradual change in plant and animal communities in an area

following a disturbance

1. Use example of old-field succession in the Piedmont Plateau to illustrate basic

concepts: (check this out next time you drive rt 13 through northern North Carolina!)

a. the disturbance, in this case, is conversion of forest to agricultural land --remove

trees, plant crops, etc.

b. because we often have a good idea of when fields were abandoned, we can

look at fields abandoned for different lengths of time to chronicle patterns of

change

c. typical sequence (there is always some variation from community to community,

even within the same general habitat type) is from community dominated by

annuals through “shrubbier” communities, to final hardwood forest

2. Some definitions:

a. primary succession = succession following disturbance severe enough to

remove soil (so includes soil formation); e.g., following glacial retreat,

volcanic eruption, severe dune erosion, etc.

b. secondary succession = succession following disturbance that leaves soil


more or less intact (note that extreme disturbance of soil may be virtually the

same as destruction/removal of soil); e.g., following fire, deforestation

c. specific sequence of community types formed during succession = sere

d. each stage in a sere = seral stage

e. first seral stage (first after disturbance) = pioneer community

f. latest seral stage = community that will persist without further change until further

disturbance = climax community

i. as a general rule (but exceptions exist!), climax communities tend to be

similar within geographic regions with similar climates, soils (they are, in

fact, the “biomes” we looked at at the beginning of the semester)

ii. within geographic regions with similar climates, soils, can still get variation

among climax communities depending on many factors, including:

a) local variation in climate, soils, aspect, topography, etc.

b) nature, frequency, and severity of disturbance

3. Succession has been well-studied in several especially important “model

systems”; we’ll use some to illustrate processes, patterns

a. Glacier Bay, Alaska: first described in 1700's; we have a good, continuous

record of changes as glaciers in the bay have retreated inland (beginning with

John Muir’s documentation from 1879)

b. dunes of Lake Michigan formed as lake levels have fallen; distance of dunes

from current shoreline is a rough measure of age (we can also use other dating

methods)

c. Hubbard Brook experimental forest (NY) -- large forest system where

investigators have been able to experimentally log and treat large areas and

follow for 30 years

d. new: Mount St. Helens has been intensively studied since eruption 20 years ago

e. old: Park Grass site in England, continuously studied for 100+ years!

f. focus is usually on plant species -- but animals follow comparable kinds of

patterns


B. Community-level changes during succession include changes in species richness and

composition

1. Species composition changes identify seral stages; we’ll look at mechanisms of

change later

a. note that change in composition includes changes in absolute presence/

absence as well as in relative abundance

2. Regardless of community, common pattern of change in species richness holds:

a. species richness increases as succession proceeds

b. rate of change is rapid initially, then slows to 0 at/near climax

c. note that pattern holds in a variety of communities over 3 orders of magnitude

difference in total time:

i. 1.5 years in intertidal communities (boulders)

ii. 150 years in Piedmont plateau communities

iii. 1500 years in Glacier Bay (why does this one take so much longer?)

d. timing of change in richness within a community is not necessarily the same for

all growth forms: e.g. from Glacier Bay: (OH)

i. mosses, liverworts reach max. richness after ~ 100 years

ii. low shrubs and herbs increase in richness throughout

C.C. Ecosystem-level changes during succession include changes in productivity,

respiration, biomass, and nutrient retention (we’ll look at some of these processes in

more detail next section)

1. Like community-level changes, ecosystem changes are often rapid initially, then

conditions stabilize at/near climax

2. Changes in soil properties are especially important in primary succession (but also

happen in 2' succession), e.g. Glacier Bay pattern = (OH)

a. increases in soil depth at all horizons

b. increases in , nitrogen, soil moisture, organic content

c. decreases in phosphorous, pH, bulk density

3. As succession increases, biomass increases


a. biomass = dry weight of organisms =~ fixed carbon

b. an increase in biomass can only happen if PSN > respiration -- i.e., plants are

fixing more energy as stored carbohydrate than plants and animals are “burning”

as fuel

c. (for this reason, early successional communities are often called carbon sinks)

d. as succession proceeds, rates of PSN stabilize; rates of respiration increase

then stabilize; at climax, the two are generally ~ balanced and total biomass is ~

stable (but there are exceptions to this!)

4. Hubbard Brook experiments demonstrate that soil nutrient retention increases as

succession proceeds:

a. very important, because (as we’ll see later) soil nutrients are often lost rapidly

following disturbance

b. experimenters logged one watershed, then experimentally suppressed

succession by applying biocides: results pretty clear (OH)

i. after succession allowed to proceed, biomass increased

ii. immediately after clearcut, nutrients were lost rapidly

iii. as succession proceeded, nutrient loss declined to control levels

D. Species turnover (which species replace which species over time) within a sere

depends on many factors:

1. biotic interactions = mechanisms of replacement:

a. facilitation = presence of one species changes the environment, making it less

favorable to original species and more favorable to a second sp.

i. e.g., light-tolerant species create shade -- reducing suitability for own

seedlings but improving conditions for shade-tolerant sp.

ii. e.g., cedars prefer limestone soils; own needles make soils more acidic,

favoring species that prefer acidic soils

b. inhibition = one species makes the environment unfavorable for other species

i. e.g., many plants use allelopathic chemicals to inhibit growth of other plants

(oaks, walnuts, e.g.)


ii. competition and predation (for animals) can also be inhibitory mechanisms

c. tolerance = lack of effect of one species on another

d. e.g., effects of dominant plants on spruce seedlings during succession at

Glacier Bay: note that any one species can have both facilitory and inhibitory

effects on another!

e. lesson from Mount St. Helens: commensalisms can also be important:

salamanders were able to survive/recolonize affected areas by living in/moving

through pocket gopher burrows via openings left by elk!

2. abiotic conditions before and during succession can influence species composition

within seral stages, particularly by affecting the species composition of the pioneer

community:

a. size of the disturbed area: in general, increasing distance from the “edge” to the

“core” of the disturbed area will increase the difference in abiotic conditions

between undisturbed and disturbed areas -- so the larger the disturbance, the

more harsh the abiotic conditons in the “most disturbed” portion and the more

different the pioneer community will be from the original

b. distance from disturbed area to source populations (sources of new seeds and

animals):

i. increased distance favors plants adapted for long-distance dispersal;

ii. decreased distance allows less dispersal-adapted plants to become part of

pioneer community

c. severity of disturbance: this can have two kinds of effects:

i. severity determines new abiotic conditions, which help determine which

species can colonize

ii. more severe disturbances may kill portions of seed bank (seeds and

propagules present in the soil), making it less likely for those species to be

included in the pioneer community

3. Mount St. Helens demonstrates complexity of succession:

a. disturbance itself was highly complex – included landslides, pyroclastic & debris


flows, ash depositions

b. recovery process did not proceed as simple pattern of seral stage replacement

from early successional pioneer to late successional forest

c. instead, got complex recovery processes that owed as much to chance as to

deterministic processes

i. except in areas of lava/hot pumice, lots of organisms (plant and animal)

survived, either as complete individuals or as reproductive parts

a) e.g., moles, gophers, salamanders persisted underground

b) roots, bulbs of some flowering plants were “tumbled” along the surface of

avalanches (rather than being buried 600 feet deep) – so could re-

establish themselves quickly

ii. many saplings and shrubs were protected intact by late-lying snowbanks

(chance = eruption happened while snowcover was still present to protect

vegetation) – these immediately resumed growth

iii. structural legacies played big role:

a) structural legacy = organic structures like blown-down trees, standing

snags, etc.

b) these have big effect on succession:

i) moderate abiotic conditions through shading, trapping moisture,

limiting erosion, etc.

ii) provide protective cover, habitat, food & other nutrient sources for lots

of species – so help those species recolonize earlier than might

otherwise be expected.

d. ON YOUR OWN: is lupine a facilitator or inhibitor of succession on Mount St.

Helens? Describe the experiments and evidence discussed on pp. 465-466.

E. Although details of species turnover vary among communities, both early and late

successional plant species have common characteristics across communities

1. Early successional species will be those that can

a. disperse rapidly over long distances


b. tolerate relatively harsh abiotic conditions

2. Late successional species will tend to be those that are good competitors over time

(note that these species are often present as seeds/seedlings in early stages and

only come to dominate later stages as they grow)

3. General scheme of characters (think about equilibrial vs. non-equilibrial species!):

characteristic early successional spp. late successional spp.

number of seeds many few

seed size small large

dispersal wind, passive animal gravity, active animal

seed viability long; can stay latent in soil short

root:shoot low high

growth rate rapid slow

mature size small large

shade tolerance low high

F. Stability in communities and ecosystems may be due to a lack of disturbance or to

resisitance and resilience in the face of disturbance

1. In lay terms, “stability” implies lack of change -- some ecological communities may

exhibit relatively little change simply because of lack of disturbance

2. More interesting to ecologists is stability even when disturbance occurs (which is

the case for most ecosystems!) -- stability can be defined as perisistence of a

community or ecosystem in spite of disturbance.

3. This type of stability can be a consequence of two different community or

ecosystem properties:

a. resistance = ability to maintain structure and/or function in the face of a

disturbance -- eg.,

i. chaparral communities are resistant to fire (at least normally!)

ii. riparian communities may be resistant to flooding


b. resilience = ability to recover original structure/function after a change following

a disturbance; succession is the process that lets this happen

4. Stability remains very poorly understood:

a. ecologists would like very much to understand correlates -- why some

communities are more stable than others; what features of community

structure/function enhance resistance and resilience

b. no general patterns yet

c. three things we do know:

i. our perception of stability may be scale-dependent: a community or

ecosystem that seems stable at one spatial, temporal, or structural scale

may be less stable at other scales:

a) e.g., Park Grass experimental site in England: from 1910 to 1948

i) at the coarsest level of resolution, fertilized and unfertilized plotswere

very stable: all began as meadows and persisted as meadows

ii) at the scale of growth forms, also get good stability: the relative

proportion of grasses, legumes, and other plants stayed very

consistent on most plots

iii) at the level of individual species abundances, found lots of variation:

some species had no change, some much, and lots in

between

b) important implication: if this kind of pattern holds following human

disturbance, how easy/hard would it be to recognize changes at the finer

levels of resolution? would that matter?

ii. communities/ecosystems that are resistant to or resilient following one kind

of disturbance might be badly affected by another

iii. stability is a function of complex interactions between biotic and abiotic

factors -- e.g., Sycamore Creek , Arizona

a) ecosystem resilience (species composition following flooding) is a

function of nitrate levels -- portions of the creek with high nitrate levels


have greatest resilience

b) nitrate levels are a function of the strength and direction of flow between

surface water and water flowing through sediments along the streambed

-- upwelling increases nitrate levels

c) spatial arrangement of upwelling, stationary, and downwelling regions is

highly resistant to flooding; it’s a function of the geomorphology of the

streambed (especially the depth of bedrock).

d) so here, geology of the streambed provides stability in one component of

community/ecosystem function; that component increases resilience in

other components (species composition)


Biology 205 - Principles of EcologyShannon-Weiner diversity index sample problem

Dr. Kilburn

In the illustration, each shape represents a different species; each individual figurerepresents an individual organism. Using the table provided, calculate H’ for bothcommunities. Which has the greater diversity? Which has the greater species richness? Which has the greater species evenness?

Community 1

Species Number Proportion (pi) logepi pilogepi

Community 2

Species Number Proportion (pi) logepi pilogepi

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